1. Industrial Field of the Invention
The present invention relates to a method for removing a substrate and a mechanism for effecting the same. More specifically, the present invention relates to a method for quickly, reliably, and safely removing a substrate from an electrode onto which the substrate is held by electrostatic clamping, and to a substrate removal mechanism for effecting the method.
2. Discussion of Related Art
According to an electrostatic clamping method, a substrate is held onto an electrode by applying a uniform electrostatic clamping force over the entire substrate. Essentially, this method is suitable for holding a large substrate. Therefore, the method of holding a substrate by electrostatic clamping is widely prevailing with the recent increase in the size of the substrates.
In holding a substrate by electrostatic clamping, in general, a DC-voltage is applied to an electrode to generate an electric potential difference between the substrate and the electrode. Furthermore, a dielectric material is interposed between the substrate and the electrode to accumulate a static charge between the substrate and the electrode. However, there is a problem in that the substrate still remains clamped to the electrode even after the application of the DC voltage to the electrode is stopped. This is due to the remnant clamping force generated by a residual static charge inside the dielectric material. It is therefore difficult to rapidly, reliably, and safely remove the substrate once it is held onto an electrode by electrostatic clamping.
Prior art techniques for removing a substrate held onto an electrode by electrostatic clamping can be generally, classified into the following two categories.
(A) Methods using a mechanical means:
(A-1) a method comprising making a pin protrude from the surface of the electrode; and PA1 (A-2) a method comprising removing the substrate from the electrode by utilizing the expansion force of a pressurized gas introduced between the substrate and the electrode through an inlet pipe provided inside the electrode. PA1 (B-1) a method comprising canceling out the residual static lingering charge inside the dielectric material provided between the substrate and the electrode by inverting the polarity of the DC-voltage; PA1 (B-2) a method comprising eliminating the clamping force by grounding both the electrode and the substrate; and PA1 (B-3) a method, in case the substrate is a semiconductor wafer, comprising eliminating the residual static charge by ceasing the application of a DC voltage during plasma generation, thereby allowing the residual static charge to dissipate. PA1 (A-1) In the method comprising the protruding of a pin from the surface of the electrode, a removal force is exerted to a local portion of the substrate with the uniform application of the electrostatic clamping force to the entire substrate. The local exertion of the removal force leads to the deformation or breakage of the substrate. PA1 (A-2) In the method comprising utilizing the expansion of pressurized gas, the substrate is sometimes blown up vigorously at the very instance the substrate is leaving the electrode. The blown-up substrate falls and may be accidentally broken. PA1 (B-1) In cases where the polarity of the residual static charge is canceled out by inverting the polarity of the applied voltage, it is practically impossible to completely eliminate the residual static charge by a single polarity inversion. Accordingly, to overcome the incomplete elimination, this method inevitably requires several polarity inversions of the applied voltage while gradually lowering the applied voltage to finally attain a voltage of zero. This polarity inversion method requires at least several tens of seconds to remove the substrate. PA1 (B-2) In the method of grounding both the electrode and the substrate, if a thin film of dielectric material (e.g,, an SiO.sub.2 film) is formed on the rear surface of the substrate the grounding method is not practical because the decay time necessary to completely eliminate the residual static charge of the dielectric film is too long. PA1 (B-3) In the method of dissipating the residual static charge by exposing a plasma to the substrate after ceasing the application of a DC-voltage, there is a fear that the temperature of the substrate will rise due to plasma heating. Furthermore, if the time duration for the plasma exposure to the substrate is set inappropriately after ceasing the application of DC-voltage the residual static charge may still remain present in a large quantity, or worse still, by the self-bias voltage which is induced at the substrate due to the plasma exposure, the substrate is charged up again. The electrostatic clamping force cannot be sufficiently lowered by the plasma exposure method.
(B) Methods using an electrical means:
However, the aforementioned prior art techniques still suffer, either in principle or in practice, from the following problems.
(A) Problems concerning substrate removal by physical force utilizing a mechanical means:
Moreover, particulate debris tends to be generated due to the friction between the pin and the substrate.
(B) Problems concerning the elimination of the residual static charge utilizing an electric means: